I don't see a reason to prep for an IQ test, though perhaps you can explain to the kid that it is unlike other tests they have taken (more like games or puzzles).
For the SAT, it is different. I am in the process of trying to get middle kid to prep for the SAT. She took it once this past year, in 10th, just to see how she would do. While her score would get her into the vast majority of colleges in the US, she wants to apply to some very selective schools. All of these schools have very low admit rates, but it would help if she got to 2250+ instead of 2100+.
My youngest did not want to prep for the MAP test, though in our district, you need to score 96th+ percentile to get accelerated in math in 5th grade. So I just told her to slow down, don't rush and make stupid mistakes, use some reasoning on questions you don't fully understand, eliminate obvious wrong answers, etc.
I don't really consider that prep or hothousing (and just that 5 minute talk led to a 98th percentile score).
